8.3.1—The terms interference and coherence

Interference is the redistribution of intensity caused when waves superpose. Coherent sources have a constant phase difference and the same frequency.

Constructive interference occurs for in-phase arrivals; destructive interference occurs for opposite phase. Track path difference or phase difference.

Two equal coherent waves can produce bright and dark regions because their displacements add or cancel at different positions.

Interference does not require waves to be identical in every detail, but stable fringes do require a fixed phase relationship.